I hope everybody isn't offended by this but I find that children who attend daycare regularly are so much more forceful than my children that it's often not even worth trying to play where there is a bunch of them. If your kid isn't the aggressor in a situation and he ends up in a fight you should be on pretty stable ground when it comes to confronting the mothers. I wouldn't back down, they're like people who don't use turn signals - totally oblivious to their affect on others, absolutely rude and dangerous.
We have encountered a couple of problems with my youngest daughter at school, she has been hit by kids at school a couple of times and spit on once. Once she was hit hard enough to leave a bruise that lasted more than a week. Her school has a 10:1 Student to teacher ratio but they still don't see everything. I told her that Toby would likely leave the class shortly (which he did) but in the mean time she was not to allow him to get away with it. She has quite a temper at times and she's quite strong so I told her that he was taking advantage of her (which made her mad) and if it happens again to either tell the teacher or to just whale on him as hard as she could muster and then go tell on him(realizing, of course, that I may have a conversation with the teacher about it later). I think she had maybe one more encounter with the main kid that was bothering her and haven't heard much from the other class brat since then either. She either got even or the bullying wore off. Her big sister was so mad she was ready to storm the primary class and beat the kid silly (and my oldest isn't violent in the least). We never had any issues with the class bullies with my oldest she is an incorrigible tattle tale, I actually had to tell her that "Its not good to tattle on your friends for minor infractions - it makes them mad at you."